1. Field of the Invention
The present invention relates to translucent polycrystalline alumina and a process for producing the same. More particularly, the present invention relates to polycrystalline alumina having improved light transmission and mechanical strength and a process for producing the same.
2. Description of the Related Art
U.S. Pat. No. 3,026,210 discloses translucent alumina and a process for producing the same, which process comprises adding 0.5% by weight or less of magnesium oxide to high purity alumina fine powder and sintering the mixture at a temperature of 1750.degree. to 1950.degree. C. in a hydrogen gas atmosphere. Further, Japanese Patent Publication No. 6831/1984 discloses a process for increasing a solubility of magnesium oxide in alumina by the addition of zirconium oxide or hafnium oxide to the high purity alumina fine powder in addition to the magnesium oxide.
According to the invention disclosed in the above Japanese Patent Publication, alumina containing at least 80% by weight of .alpha.-alumina is used, and about 0.03 to 0.15% by weight of magnesium oxide and about 0.002 to 0.07% by weight of zirconium oxide or about 0.003 to 0.07% by weight of hafnium oxide are added to alumina, and the mixture is molded and calcined at a temperature of about 1750.degree. to 1950.degree. C. in a hydrogen gas atmosphere, whereby the amount of magnesium oxide is not substantially decreased during calcination.
As is well known, the presence of magnesium oxide will accelerate elimination of pores from grain boundaries of alumina in a sintering step and suppress abnormal growth of grains, whereby uniform crystalline grains are formed. If magnesium oxide is not present or its amount is too small, no polycrystalline alumina having good light-transmission is obtained.
When an excess amount of magnesium oxide is added to alumina, a spinel phase consisting of magnesium aluminate is formed on the grain boundaries of alumina grains, so that the light transmission of alumina is decreased.
It is known that added magnesium oxide is dissipated and decreased in the high temperature calcination step, and the dissipation rate depends on various parameters such as the shape of the molded material, the heating rate, the holding time, and the size or shape of the sintering furnace. Thus, it is very difficult to control the process parameters in the commercial production of translucent alumina.
Since R. L. Coble introduced the light transmission property of polycrystalline alumina, many improvements have been proposed. When the translucent polycrystalline alumina is used in an application such as an arc tube of a sodium discharge lamp, only a slight improvement of light transmission of polycrystalline alumina causes improvement in the luminance of the sodium-discharge lamp and increases the commercial value of such lamp greatly. Accordingly, polycrystalline alumina having higher light transmission is still required. In addition, when the lamp is on or off, the tube may be broken by thermal shock, polycrystalline alumina having high strength.